Stabilization and solidification of hazardous waste material are described in "Handbook for Stabilization/Solidification of Hazardous Waste" produced by the United States Environmental Protection Agency at the Hazardous Waste Engineering Research Laboratory, Cincinnati, Ohio and published in June of 1986 (hereafter called "June 1986 EPA Report"). Said June 1986 EPA Report is hereby incorporated by reference in its entirety.
The terms "stabilization" and "solidification" are used herein as in said June 1986 EPA Report. Stabilization techniques are generally those whose beneficial action is primarily through limiting the solubility or mobility of the contaminants in hazardous waste with or without change or improvement in the physical characteristics of the waste. Stabilization usually involves adding materials which ensure that the hazardous constituents are maintained in their least mobile or toxic form. Solidification implies that the beneficial results of treatment are obtained primarily, but not necessarily exclusively, through the production of solid waste material which has structural integrity. The contaminants do not necessarily interact chemically with reagents, but are mechanically locked within the solidified matrix and/or bonded to or surrounded by an impervious covering. Some of these techniques for treating hazardous wastes are also considered to be stabilization/solidification processes. The term "fixation" is often used in the waste treatment field to mean any of various stabilization/solidification processes such as described in said June 1986 EPA Report; "fixed" wastes are those that have been treated by such fixation processes.
Free liquids in solid wastes are the liquids which readily separate from the solid portion of a waste under ambient temperature and pressure. Current regulations prohibit disposal of solid waste containing free liquids without treatment by mixing with an absorbent material. Most waste materials considered for stabilization/solidification are liquids or sludges (semisolids). To prevent the loss of drainable liquid and improve the handling characteristics of the waste, a dry solid sorbent is generally added to the waste. The sorbent may interact chemically with the waste or may simply be wetted by the liquid part of the waste and retain the liquid. The most common sorbents used in treating hazardous waste include soil and waste products such as bottom ash, fly ash, or kiln dust from cement and lime manufacture. In general, selection of sorbent materials involves trade-offs among chemical effects, costs, and amounts required to produce a solid product suitable for in-situ capping or burial. Though some sorbents are relatively inert, undesirable and even hazardous reactions can and do occur unless attention is paid to the potential for waste and sorbent to react.
Several generic treatment systems have been developed for waste stabilization and solidification. The volumes of waste involved at uncontrolled waste sites generally require use of only the least expensive systems that are effective. The large quantities and varieties of wastes that are usually present also require the use of adaptable systems that are effective over a wide range of conditions. Four alternative onsite stabilization/solidification systems are being used: (1) in-drum mixing, (2) in-situ mixing, (3) mobile plant mixing, and (4) area mixing. The present invention relates to in-situ solidification/stabilization mixing.
In-situ mixing is primarily used for closure of liquid or slurry holding ponds or lagoons. In-situ mixing is most applicable for the addition of large volumes of low reactivity solid chemicals. In-situ mixing has been used primarily for the treatment of low solids content slurries or sludges. Where applicable, in-situ mixing is usually the lowest cost alternative. However, quality control associated with in-situ mixing has been limited with the technology heretofore available. Further, in-situ mixing has been the most difficult hazardous waste treatment alternative in terms of control of safety and environmental considerations. Since the entire process is open to the atmosphere, problems include the generation of fugitive dust, odors, and vapors. It is required to minimize exposure of personnel and equipment to the waste materials being treated and dust, fire or other hazards, depending on the waste being handled. It is important to minimize reduction in production efficiency by reducing the extent and interference of protective apparatus and procedures employed to provide the level of protection meeting pertinent regulations.
An effective in-situ hazardous waste treatment system is desirable because it is the fastest and least expensive of the alternative treatment systems noted above. The speed and economy possible with an in-situ waste treatment system are largely due to the reduction in the amount of handling of the waste mass. Other than for mixing, the wastes are usually moved only once; or if the wastes are not too hazardous, they are often not even removed from the original waste lagoon but mixed and left in place. This method lends itself best to liquid or low-solids sludges which are easily mixed. Heretofore, heavy sludges have been mixed with heavy equipment like draglines or clamshells, but with a problem obtaining sufficient uniformity in the treated product. Major limitations of the prior known systems for in-situ treatment of hazardous wastes are the low amount of mixing attained and the inability to control accurately the proportion of reagent to waste which can result in a nonuniform, unevenly mixed final product. This can be overcome to some extent by using excess reagent to decrease zones of low reagent content, but that increases cost.
The simplest commercial solidification/stabilization system for in-situ mixing of hazardous wastes uses common construction machinery (typically a backhoe, bull dozer, or pull shovel) to accomplish the mixing process. Where large lagoons are being treated, clamshells and/or draglines have also been utilized. This technique is used for application to liquids or light flowable sludges having a high liquid content. The technique also is used for solidification/stabilization processes incorporating the addition of large amounts of bulk powdery solids (kiln dust, fly ash, etc.) to the waste materials. However, there are problems in obtaining uniform satisfactory mixing and good stabilization/solidification results with these prior systems. There are also problems of dust and other hazards. (See said June 1986 EPA Report, especially page 6-10.)
Two prior pieces of equipment which are designed and used specifically for in-situ waste mixing have been introduced commercially (see FIGS. 6-4 and 6-5 of said June 1986 EPA Report at page 6-16 and related discussion at pages 6-15 to 6-18 thereof). This equipment is mounted on the boom of a backhoe or like construction equipment. These systems pneumatically inject the reagent directly into the waste mass at the lower end of multiple elongated spaced cylinders which are used to stir and mix the wastes. Fly ash is delivered to the multibarreled injection head via a compressed air system. Hydraulically driven augers in the lower section of each of the multiple spaced barrels force the fly ash out of the barrels into the basin waste contents. As fly ash is forced from the barrels into the waste, the boom simultaneously moves the injection head back and forth (in the plane of the boom) as well as up and down to mix the fly ash and basin contents. Pneumatic pressure on the fly ash delivery to the injection heads in this system causes loss of fly ash to the air at the basin surface which results in bursts of fly ash dust (or other reagent) which is objectionable.
This equipment has certain disadvantages; the auger blades were subject to fouling or clogging, the mixing achieved was not as complete as desired, and a high torque was required for mixing. Providing a substantially uniform mixture is important especially if the mixture is exothermic. The heat generated by a nonuniform mixture could result in spontaneous combustion, undesirable chemical breakdown, or the like.
The addition of a dry powdery material such as flyash or lime kiln dust can present a problem. The addition of the material to a lagoon can cause an exothermic reaction which could result in a fire. It is then desirable to mix the solid material with sufficient quantities of the liquid to dissipate heat generated. Furthermore, the dry material itself can be an airborne hazard and, therefore, it must be incorporated in the lagoon without creating an unacceptable amount of airborne dust.
Depending upon the type of lagoon, it may be desirable to inject liquids, gases, or slurries in addition. For example, it may be desirable to inject liquids such as peroxides to oxidize contaminants, or gases such as carbon dioxide to facilitate the formation of carbonate salts. It also is possible to inject slurries such as an activated carbon slurry for absorbency, and odor control.
The dry material added could be a material such as type C flyash, lime kiln dust, cement, or the like. Obviously then in the case of flyash or lime kiln dust, a hazardous dry material can be used in disposing of a lagoon also containing hazardous materials. Sufficient dry material should be added and mixed with the free water or liquid in the lagoon so that the mixture will gel or set within three days. The lagoon can either be capped and left in place or removed by truck to an appropriate landfill for spreading.